The invention relates to a tandem ion accelerator having a matter-free ion charge reversal zone.
To a rapidly increasing extent, in research and also in industry, high-speed i.e. high-energy ions are being used for doping solid bodies with foreign elements for use in medicine and for the investigation of reactions between energy particles in various states of aggregation etc. In accordance with the current prior art, high-speed ions are mainly obtained via linear accelerators, ring accelerators and electron ring accelerators.
In industry, in the medium energy range, linear accelerators are mainly employed in the field of ion implantation. Simple linear accelerators, for example, operating in accordance with the Van de Graaff principle, are used most frequently. These accelerators consist of a simple, one-element or multi-element acceleration path. Generally the ion source carries high voltage, the object ground potential, and the mass separator carries either ground potential or high voltage. Other more complicated modes of operation have not been adopted.
If acceleration voltages in the MeV range are required, the components connected to high voltage and the acceleration path are operated in high pressure tanks in order to considerably reduce the large safety clearances with respect to sparkovers which are usually required at atmospheric pressure and which are intolerable. The disadvantage is accepted that the ion source, which requires frequent servicing, can only be reached with great expense. An often desirable preliminary decomposition of the ion beam is likewise virtually impossible. High current ion sources are therefore not currently used in systems of this kind.
The problem that the sensitive components of an acceleration system are connected to high voltage and arranged in a pressure tank can be avoided by means of tandem accelerators. In a tandem accelerator, an injector (ion source and mass separator) connected to ground potential emits negative ions into an acceleration path, whose end thus carries high voltage. In the adjoining gas or solid body target these negative ions are reversed in charge to form positive ions as a result of collisions with the target atoms through the withdrawal of electrons. In a second adjoining acceleration path whose end now carries ground potential, the now positive ions are again accelerated and thus obtain double energy. The ions can again be separated. The irradiation object is connected to ground.
A disadvantage of this arrangement consists in that the yield of negative ions at the injector output is low and that the interaction between the negative ions and the charge reversal target produces different positive charge states with an energy spread in comparison to the oncoming beam and with an increase in the beam divergence. This means a reduction in the beam intensity. Therefore in many respects tandem accelerators are to be preferred to simple accelerators, but on the other hand they supply only low ion currents.
Further disadvantages of conventional tandem accelerators result from the fact that the interaction between the originally negative ions and the neutral or charged atoms or molecules of the matter of the charge reversed target results in a distribution between the various possible positive charge states. Therefore in practice one is generally left with a single-energy particle beam, and must consequently filter ions of one single charge state out of the ions of different charge states. The filtering process reduces the ion yield. The interaction of the ions in the charge reversal target gives rise to scatter processes so that the particles are deflected from their original path. This results in a spread of the original bunch of beams. This spread likewise reduces the beam intensity. Under certain circumstances the normal widening of the ion energy caused by the scatter process can also prove disturbing. Fixed charge reversal targets consist of thin foils which rapidly become unserviceable so that the pressure chamber must frequently be opened in order to exchange the foils. Gas targets involve vacuum problems, i.e. evacuation problems also manifest by the undesired formation of intensive x-rays produced by the secondary electrons.